I would have a global memory tag that is the timestamp of when the door opened. You can have another tag with an expression along the lines of
max(0, dateDiff({[default]PathToDoorOpenTimestamp}, now(), 'second')).
From there you can then use the value of that expression tag to do additional formatting to get your time elapsed as HH:mm or mm or ss.
For your timer display, you can set an expression on the display property that is {[Path]ToTimeElapsedTag}>300, and then have that label display the formatted time elapsed value.
should help some with figuring out how to get the seconds elapsed value into the format that you want